The present invention relates to an optical scanning device for a laser beam printer or the like. In particular, the present invention relates to an optical scanning device utilizing an SOS (Start Of Scan) signal as a synchronizing signal for adjusting a timing of a scanning beam.
Conventionally, in an optical scanning device such as an LSU (Laser Scanning Unit), the SOS signal is obtained as follows.
In the conventional LSU, the scanning laser beam is received by a photo diode, which outputs a light detection signal. Based on the light detection signal, the SOS signal is generated. Specifically, in the LSU, a laser beam LB is emitted by a laser diode. The laser beam is deflected by a polygon mirror which rotates at a relatively high speed to deflect the laser beam to scan within a predetermined angular range. The scanning beam is directed onto a photoconductive drum via an fxcex8 lens and scans the circumferential surface of the photoconductive drum in a direction of an axis thereof from one side to another (i.e., in a main scanning direction). While the beam scans in the main scanning direction, the photoconductive drum rotates about the axis thereof (i.e., an auxiliary scanning is performed). Within a scanning range of the laser beam, but out of an image forming area of the photoconductive drum, a photo diode is arranged. The photo diode receives the scanning beam and outputs a light detection signal, which is transmitted to the SOS signal processing circuit. Based on the SOS signal output by the SOS signal processing circuit, a controller controls an LD drive circuit. If the image formation is executed a predetermined period after the SOS signal has been output (i.e., the image formation is executed synchronously with the SOS signal), the image is always formed in the imaging area on the photoconductive drum.
In the photo diode for obtaining the SOS signal, received amount of light may vary due to variation of output power of the laser beam, oscillation of surfaces of the polygon mirror or the like. Therefore, the level of the light detection signal output by the photo diode may not-be constant. Further, the level of the light detection signal may also vary due to noise introduced at the photo diode and/or the signal processing circuit, a deviation of DC component of the light detection signal due to the dark current, and while the signal is transmitted from the photo diode to the signal processing circuit.
If the SOS signal is generated based on such an unstable light detection signal including noises, the timing of the SOS signal may vary, and it is difficult to obtain a reliable SOS signal.
In particular, if the conventional SOS signal generating circuit generates the SOS signal by comparing the light receiving signal value with a predetermined threshold value, the variation of the signal level directly affects the timing of the SOS signal.
It is therefore an object of the invention to provide an SOS signal generating circuit which is capable of suppressing noises in the light receiving signal and generating a reliable SOS signal.
According to an aspect of the invention, there is provided an optical scanning device, provided with:
a light source;
a scanning system which deflect the beam emitted by the light source to scan;
a light receiving unit having a plurality of light receiving elements arranged in a direction in which the scanning beams scans, a light receiving signal being output by each of the light receiving element upon incident of the scanning beam, the plurality of light receiving elements being divided into two groups such that every other light receiving element is fallen in the same group and adjoining light receiving elements are fallen in different groups;
a resonance amplifying system that amplifies the light receiving signals of the light receiving elements of the two groups, respectively;
a clock signal generating unit that generates a clock signal in accordance with the resonance amplified signals;
a delay signal generating circuit that generates a delay signal for generating a synchronizing signal in accordance with the light receiving signals output by at least part of the plurality of light receiving elements; and
a logic circuit which generates the synchronizing signal based on the clock signal output by the clock signal generating unit and the delay signal output by the delay signal generating unit.
The light receiving signals corresponding to each group are synthesized and resonance-amplified, and the clock signal is generated therefrom. Further, the delay signal which defines the start of the synchronizing signal is generated based on the light receiving signals. Then, the synchronizing signal is generated using the stabilized portion of the clock signal. Accordingly, a reliable SOS signal can be obtained.
Optionally, the scanning optical system may include:
a first adder that synthesizes the light receiving signals output by the light receiving elements fallen in one of the pair of groups; and
a second adder that synthesizes the light receiving signals output by the light receiving elements fallen in the other one of the pair of groups.
In this case, the resonance amplifying system may include:
a pair of narrow-band amplifiers that resonance amplify signals respectively output by the first and second adders; and
a first comparator for comparing the resonance amplified signals output by the pair of narrow-band amplifiers, output signal of the first comparator being utilized as the clock signal.
Further optionally, the resonance frequencies of the pair of narrow-band amplifiers are substantially coincide with the frequency of the sum of the light receiving signals output from the first and second adders, respectively.
Still optionally, the delay signal generating unit may include:
a second comparator that compares the sum of the light receiving signals output from all of the plurality of light receiving elements with a predetermined reference level; and
a first timer circuit that is triggered in response to an output signal of the second comparator to output a delay signal for a predetermined period.
Further, the delay signal generating unit may include:
a third comparator that compares the light receiving signals output by first two of the plurality of light receiving elements;
a fourth comparator that compares the sum of the output signals of the first two of the plurality of light receiving elements with a predetermined reference level;
an AND gate that applies AND operation to the outputs of the third and fourth comparators;
a third timer circuit that is triggered in response to an output signal of the AND gate to output a delay signal for a predetermined period.
Further optionally, the logic circuit unit may be provided with;
an enabling signal output system that outputs an enabling signal upon end of the delay signal output by the delay signal generating unit;
a second timer that outputs, in accordance with the enabling signal and the clock signal output by the clock signal generating unit, a gate signal which is kept for a predetermined period; and
a logic gate that outputs the synchronizing signal when the clock signal and the gate signal are input.
Optionally, the delay signal output by the delay signal generating unit is kept output at least when the light receiving signals are output by the plurality of light receiving elements.
In particular, the delay signal is terminated by a point of time at which an amplitude of the output signal of each narrow-band amplifier is a half of the maximum value thereof.
Further, the delay signal output by the delay signal generating unit is terminated within a rage from one period before to one period after with respect to a point of time where the output of the resonance amplifier has a maximum value.
According to another aspect of the invention, there is provided an optical scanning device, provided with:
a light source;
a scanning system which deflect the beam emitted by the light source to scan;
a light receiving unit having a plurality of light receiving elements arranged in a direction in which the scanning beams scans, a light receiving signal being output by each of the light receiving element upon incident of the scanning beam;
a resonance amplifying system that amplifies the light receiving signals of the light receiving elements;
a clock signal generating unit that generates a clock signal in accordance with the resonance amplified signals;
a delay signal generating unit that generates a delay signal for generating a synchronizing signal in accordance with the light receiving signals output by the plurality of light receiving elements; and
a logic circuit which generates the synchronizing signal based on the clock signal output by the clock signal generating unit and the delay signal output by the delay signal generating unit.
The light receiving signals are synthesized, and then resonance-amplified. Then, based on the resonance-amplified signal(s), a clock signal is generated. Utilizing the delay signal, detection of the clock signal is delayed until the clock signal is considered to be stabilized. Then, in accordance with the clock signal, the SOS signal is generated. Thus, a reliable SOS signal can be obtained.
Optionally, one of the plurality of light receiving elements which is firstly scanned by the scanning beam may be constructed to have wider than the other light receiving elements.
Further optionally, among all the light receiving elements, one of every predetermined number of light receiving elements is used for outputting the light receiving signal.
Furthermore, the scanning optical system may be provided with an adder that synthesizes the light receiving signals output by the light receiving elements, and the resonance amplifying system may include: a narrow-band amplifier that resonance amplify signals output by the adder; and a first comparator for comparing the resonance amplified signals output by the of narrow-band amplifier with a predetermined reference level, output signal of the first comparator being utilized as the clock signal.
Preferably, the resonance frequency of the narrow-band amplifier substantially coincides with the frequency of the synthesized signal of the light receiving signals output from the adder.
In particular case, the delay signal generating unit may be provided with: a second comparator that compares the sum of the light receiving signals output from the plurality of light receiving elements with a predetermined reference level; and a first timer circuit that is triggered in response to an output signal of the second comparator to output a delay signal for a predetermined period.
In this case, delay signal output by the delay signal generating unit may be kept output at least when the light receiving signals are output by the plurality of light receiving elements. Further, the delay signal may be terminated by a point of time at which an amplitude of the output signal of the narrow-band amplifier is. a half of the maximum value thereof. Alternatively, the delay signal output by the delay signal generating unit may be terminated within a rage from one period before to one period after with respect to a point of time where the output of the resonance amplifier has a maximum value.
Further optionally, the logic circuit unit may include: an enabling signal output system that outputs an enabling signal upon end of the delay signal output by the delay signal generating unit; a second timer that outputs, in accordance with the first gate signal and the clock signal output by the clock signal generating unit, a gate signal which is kept for a predetermined period; and a logic gate that outputs the synchronizing signal when the delay signal and the gate signal are input.
According to another aspect of the invention, there is provided an optical scanning device, which is provided with:
a light source;
a scanning system which deflect the beam emitted by the light source to scan;
a light receiving unit having a plurality of light receiving elements arranged in a direction in which the scanning beams scans, a light receiving signal being output by each of the light receiving elements upon incident of the scanning beam;
a signal adding system that adds the light receiving signals output by the plurality of light receiving elements to generate an added signal;
a clock signal generating system that generates a clock signal based on the added signal, the clock signal corresponding to a scanning speed of the scanning beam;
a delay signal generating unit that generates a delay signal defining a delay period during which the clock signal is ignored, the delay signal being generated based on light receiving signals output by a part of the plurality of light receiving elements which are located on upstream side of a scanning direction of the scanning beam; and
an SOS signal generating circuit which generates a synchronizing signal based on the clock signal and the delay signal.
Optionally, the clock signal generating system may include: a resonance amplifier that resonance-amplifies the added signal; and a comparator that compares an amplitude of output of the resonance amplifier with a predetermined reference value, and outputs a comparison result, the SOS signal generating circuit is allowed to output the SOS signal after the delay signal has been received, and the delay signal may generate the delay signal such that output of the SOS signal by the SOS signal generating circuit is inhibited until the output of the resonance amplifier would be stabilized.
In this case, the SOS signal generating circuit may generate the synchronizing signal in response to a rising edge of the clock signal which immediately comes after a falling edge of the clock signal which comes after the end of the delay signal.